Lumped element antenna couplers have been used in the past to efficiently couple energy into antennas whose impedance is not matched with that of the transmission line. Typically, transmission lines are 50-ohm devices and when using, for instance, whip or monopole antennas, these antennas typically have impedances at the base of the antenna at about 0.05 ohm in the high frequency or HF band. When the transmission line is matched to the impedance at the base of the antenna, the coupler limits the energy dissipated in resistive losses and maximizes the transmitted energy. so that the antenna can be easily excited and operated at or near resonance.
Thus, antenna couplers for monopole antennas are able to match the impedance at the feed of the antenna with that of the transmission line by raising its relatively low impedance to that of 50 ohms.
Moreover, not only does one have 0.05 ohms at the base of a whip or monopole, one has a relatively large reactance which must be canceled out for efficient transfer of energy from the signal source to the antenna. While the large reactance might typically be canceled out using a loading coil to cancel out the capacitive reactance, one nonetheless has to match whatever resistance is left to the 50-ohm impedance of the transmission line.
Typical antenna couplers in the past involving lumped elements include combinations of inductors and capacitors in either a T network or a pi network configuration. In order to change the inductance or capacitance so that the impedance at the feed of the antenna is matched to the impedance of the transmission line, originally inductors were mechanically tapped along their coils or capacitors were provided with variable capacitance plates. The inductance and the capacitance were varied mechanically in order to match antenna impedance to the impedance of the transmission line. However, for frequency-hopping applications in which the frequency of the source is switched in microseconds, it became necessary to utilize solid state switching in order to switch in or out various taps of a coil or in order to switch various capacitors in and out in time to accommodate the frequency change.
Problem with the utilization of such lumped elements center around I2R ohmic heating losses which result either from the relative thinness of the wire utilized in the inductors or internal resistance of the solid state switches. Moreover, since the solid state switching utilized in these couplers was placed at high-current nodes, oversized and expensive switches were required.
Thus, while mechanical switching was suitable some 40 years ago for antenna couplers, with the advent of frequency-hopping, it is too slow. There was therefore a need for rapid re-tuning of antenna couplers that required the use of solid state switching.
Regardless of whether solid state switches were used, the prior lumped element couplers resulted in I2R losses that in turn resulted in a 20 to 30 dB reduction in radiated power.
Since large amounts of power are lost in the inductors and the diode switches that switch in and out the capacitors and inductors, one requires a much more efficient coupling system. The ohmic losses are primarily due to the circulating currents in the elements that can rise to huge values to cause the high I2R ohmic losses. The only way one could reduce these losses with conventional lumped element couplers is to make the inductors and capacitors very big. One would therefore have to have a container that was perhaps three feet by three feet by three feet in order to attempt to limit the, ohmic losses. However, as one makes the inductors large, the Qs get too high, which results in extremely high voltages and even greater ohmic losses. Additionally, with the very high voltages involved with the large components, the diodes that are utilized in the solid state switching are heavily stressed. Thus, solid state switches for these larger units would have to be extremely massive and expensive.
For a reasonably-sized lumped element coupler operating, for instance, at 2 megahertz and feeding, for instance, a loop antenna having a six-foot radius corresponding to a whip on the back of the truck which has its tip bent and attached to the forward end of the truck, gains have been measured at −23 dBi. This means that a considerable amount of the power which should be coupled to the antenna is lost as heat in the antenna coupler. As will be seen hereinafter, substituting a switched meander line architecture for the lumped element coupler results in a −13 dBi overall gain, which is an improvement of 10 dB over the lumped element coupler. This corresponds-to an order of magnitude improvement.